Maximum thrust and efficiency of a gas turbine engine is achieved when the exhaust passes through a discharge nozzle which controls expansion, and maximizes the discharge velocity. When an aircraft operates at both subsonic and supersonic speeds the exhaust nozzle pressure ratio varies over a substantial range.
Under subsonic flight conditions the pressure ratio is relatively small and a nozzle having a substantially convergent shape is desirable. At supersonic flight conditions when the nozzle pressure ratio is high the appropriate geometry is achieved by a nozzle having a convergent portion followed by a divergent portion. This is referred to as a convergent/divergent nozzle.
Many designs have been made which provide the variable geometry which will effect proper operation at both subsonic and supersonic speeds. For the subsonic condition where only the convergent nozzle is desired there is only a nominal divergence, this being selected to assure that the throat area remains upstream of the divergent flaps. At supersonic speeds, means are supplied to effect the appropriate divergent flowpath downstream of the throat. Many of these designs require additional structure and weight to achieve the actuation of the divergent flaps. It is apparent that for an aircraft engine light weight and simplicity are desirable features. It further is useful to have a structure which may be easily sealed against leakage.
In modern military aircraft additional maneuverability is desirable. This may be achieved by using pitch and yaw vectoring which requires discharging the exhaust gas selectively with a velocity component other than straight back. Again simplicity and lightweight is desirable in achieving this vectoring. It is also desirable that the structure used have a minimum impact on the external configuration of the nozzle and aircraft.